Raw coal which has been removed from a coal mine is generally referred to as run-of-mine coal and comprises coal and noncoal material. The noncoal material is generally referred to as ash and comprises pyrite clays and other aluminosilicate materials. If these noncoal ash materials are left in the run-of-mine coal, they create problems during combustion, such as slagging, fouling and a general decrease in combustion efficiency. In addition, the ash materials create pollution problems when burned with the coal. In particular, burning of coal with high ash content generates sulfur dioxide, which is typically required by law to be removed by utilities which burn the coal. Specifically, laws, such as the Clean Air Act in the United States, place limitations on the amount of sulfur dioxide which can be emitted by such facilities.
One way to reduce emissions and alleviate other environmental concerns is to remove the noncoal material from the run-of-mine coal prior to combustion. “Beneficiation” refers to the removal of noncoal material from raw coal to produce a relatively clean coal product. Processes for the beneficiation of coal may generally be classified as either wet processes or dry processes. Currently, wet beneficiation processes are the most predominant in industry. These processes use either water or other liquid materials in a manner that takes advantage of the difference in density of the coal and ash materials in order to separate the coal from the ash. In these wet processes, the run-of-mine coal must generally be pulverized into relatively fine coal particles in order to effectuate adequate separation of the coal and ash.
Dry beneficiation processes also take advantage of the differences between the densities of the coal and ash to clean the coal, but without utilizing water. Conventional dry beneficiation processes generally utilize a fluidizing bed, containing a fluidizing media (such as magnetite) with a density intermediate the coal and ash materials, to stratify a mixture of run-of-mine coal and the media into layers of coal and ash using pressurized air. In some arrangements, the fluidizing bed is also vibrated to take further advantage of the density differences while cleaning the coal. One drawback of these prior dry beneficiation processes is that the fluidizing media must generally be separated from the cleaned coal subsequent to removing the ash.
Wet processing has generally been utilized over dry processing methods because, heretofore, it has been difficult to obtain high calorific values for coal which has been beneficiated in a dry process. The caloric value of coal is a measure of the combustion efficiency. The wet processes, however, also have various drawbacks. Wet processing, for example, necessarily adds moisture to the beneficiated coal. This moisture decreases the combustion efficiency, or calorific value, and the wet processed coal must generally be dried prior to combustion. The additional steps and apparatus required to dry the wet processed coal increases the overall cost of the process. Added moisture to the coal also makes the coal susceptible to freezing in cold climates. On the other hand, in areas where the climate is very dry, water may not be readily available or there may be prohibitions against using water for applications where the water cannot be added back to the water cycle.
Wet processing methods also suffer from various handling issues. Because the run-of-mine coal must be pulverized to a very small size, wet processes may not be effective for cleaning extremely fine coal and pyrite particles due to surface phenomenon which interfere with the separation process. Furthermore, very small coal particles are harder to dry in mechanical processes, which generally utilize pressurized air. Fine particles of wet coal are also difficult to transport through automated machinery and to handle in bulk. Finally, the equipment outlay for wet processing of coal is generally more expensive compared to the equipment outlay required for dry processing of coal. Perhaps the most significant drawback of wet beneficiation of coal is the environmental impact, namely the generation of sulfuric acid as a bi-product of the process.
There is thus a need for an apparatus and method of beneficiating coal in a dry process which results in a coal product that exhibits sufficiently high calorific value and which overcomes drawbacks of the prior art such as those mentioned above.